Cryogenic liquids, such as nitrogen, argon, nitrous oxide, oxygen, carbon dioxide, hydrogen, and the like, liquify at extremely cold temperatures. Unique problems are encountered in handling and storing cryogenic liquids because the liquids undergo density changes at various storage pressures. A cryogenic storage system contains an insulated tank for containing a cryogenic liquid in a liquid space. Even though the tank is insulated, heat will enter the tank, causing the liquid cryogen to slowly vaporize to a gas and, as a result, causing the volume of liquid in the tank to diminish. This vaporization creates a pressurized head space in an upper portion of the tank.
Differential pressure gauges and sensors are well known in the art for aiding in monitoring the volumes of liquids. A differential pressure sensor senses the difference between a pressure at the head space of the tank, or head pressure, and a pressure at the liquid space of the tank, or liquid pressure, also known as column pressure. The liquid pressure is affected by both the pressure created by the head space of the tank and the pressure due to the weight of the liquid in the liquid space above the liquid space measuring point. By measuring the pressure difference between the pressure at the liquid space and the pressure at the head space, the differential pressure sensor senses the pressure solely attributable to the weight of the liquid. Typically, this pressure is measured either in pounds per square inch (psi), or in inches of water column.
By dividing the sensed differential pressure by the density of the liquid, the height of the liquid above the liquid space measuring point may be calculated. This liquid height can then be displayed on the gauge. Determining the volume of the liquid in the container is more difficult, however. Once the differential pressure has been measured, an operator must turn to a calibration chart, separate from the gauge, to determine the liquid volume. Calibration charts are also required in order to determine a total liquid weight, or an equivalent gas volume (typically measured in standard cubic feet). The relation between the differential pressure measured by the sensor and the liquid volume of the tank is affected by the tank shape, dimensions, and orientation, as well as the liquid density. Each calibration chart is therefore uniquely designed for a particular cryogenic tank model, tank orientation, type of cryogenic liquid and expected pressure of the liquid. The liquid density is a function of the liquid type and the state of its pressure. In order to determine a liquid volume level, the operator must procure an appropriate chart and use the differential pressure reading with the chart. Such calibration charts are awkward to use, and separate charts are required for different combinations of the factors listed above. This prevents efficient on-site monitoring of the liquid volume.
There is a need in the art for a method of determining, in real-time, a liquid volume using an on-site differential pressure gauge.
There is a further need in the art for a differential pressure gauge that does not require the use of calibration charts in order to determine a liquid volume.